FIELD OF THE INVENTION
The invention relates to a method for reducing harmonics mains-power supply perturbations of a self-commutated multilevel inverter installation which is clocked at the fundamental frequency and has two multilevel inverters with capacitive storage devices. The invention also relates to an apparatus for carrying out the method.
The structure of a number of self-commutated multilevel invertors which are clocked at the fundamental frequency and have capacitive storage devices is explained in German Utility Model G 94 16 048.1, corresponding to U.S. application Ser. No. 08/512,595, filed Aug. 8, 1995, in which the method of operation thereof is described in detail. A multilevel invertor of that type is used, for example, in a power factor correction device, which is referred to as "Static Var Generator (SVG)" or as "Advanced Static Var Compensator (ASVC)" or as "Static Condenser (STATCON)".
The spectrum of the three-phase invertor output voltage can be varied by selection of turn-on and turn-off instants of individual voltage levels of the multilevel invertor. The more voltage levels the multilevel invertor has, the more harmonics in the three-phase invertor output voltage can be suppressed. Thus, for example, the 11th, 13th, 23rd, 25th and 35th harmonic can be suppressed with eleven voltage levels of a twelve-pulse multilevel invertor. Consequently, the lowest harmonic which is then output into a three-phase mains power supply is the 37th harmonic. A multilevel invertor of that type produces an invertor output voltage which has a specific fundamental content.
The fundamental of the three-phase invertor output voltage defines a terminal voltage of the multilevel invertor. In practice, that determines the turns ratio of the converter transformer.
Optimum system tuning with minimum harmonics mains-power-supply perturbation consequently exists for an operating point of the multilevel invertor.
If the turn-on and turn-off instants of the individual voltage levels are then altered, the terminal voltage of the multilevel invertor is also changed. Due to that change, a mains current flows having an amplitude that has increased and a time characteristic which is distorted. Such a current can cause the multilevel invertor to turn off and can thus lead to an abrupt change in the reactive power in the three-phase mains power supply.